Leveling up this ability from lv.4 to lv.5, with a 3.500-ish max MATK weapon equipped, gave a bonus around 60~70, if my memory serves me right. Which seems to go pretty well with my assumptions, seeing how 2% of 3.500 are exactly 70. Of course I could be making these numbers up, but please trust me, it really happened as I tell you it did ^_^
This makes Memorandum of Blood a reasonably powerful passive, although still costing 125 skill points. To be noted is the fact that a great part of the useful spells of this class scale from total MATK, and as such see their damage increase as this passive is leveled up.